Si3N4 powders mixed with 6wt%Y₂O₃-2-6wt%Al₂O₃ and 6wt%Y₂O₃-1.5wt%MgO as sintering aids, were pressed and normally sintered at 1600°C. The sintered compacts were HIP'ed without capsule using in 100 MPa nitrogen gas at 1700°C. Density, weight change, a to β phase transformation and flexural strength were studied, and the results obtained were summarized as follows:
(1) The amount of weight loss during sintering was relatively small in case of Y₂O₃-Al₂O₃ series, so that the sinterability was better than in Y₂O₃-MgO series.
(2) In case of both the series, the amount of a to β transformation in sintered compacts was decreased compared with the compacts which contained 6 wt%Y₂O₃ only, and the tendency in Y₂O₃-Al₂O₃ series was more remarkable than in Y₂O₃-M90 series.
(3) The mean strength of Y₂O₃-Al₂O₃ series was improved from 340 to 700 MPa by HIP treatment. It was found that the improvement of strength depended on not only density, but also a toβ transformation and fibrous structure formation in Si₃N₄ during HIP treatment.